1. Field of the Invention
This invention pertains to magnetic cylindrical (bubble) memory devices and more particularly to a more closely packed arrangement of such devices, means of fabrication of such arrangement and means for manipulating and otherwise handling of such an arrangement.
2. Description of Prior Art
The term "bubble" sometimes referred to as a "single wall domain," refers to a magnetic region encompassed by a single domain wall which closes on itself in the plane of the material in which the domain is moved. Inasmuch as a domain is self-contained in the plane of the material and does not intersect the edge of the material, it is free to move in that plane in response to consecutively offset fields. A domain of this type is described in the Bell System Technical Journal (BSTJ) Vol. XLVI, No. 8, (1967) at page 1901.
The fields which move single wall domains are typically generated by pulses in series-connected conductor loop sets. The sets are pulsed in sequence to generate repetitive field patterns for moving information-representative domain patterns. Individual conductors can be pulsed also in a programmed manner to effect data processing functions between selected domains. A conductor arrangement of this type requires a number of external connections which are, preferably, kept to a minimum.
An alternate implementation for generating the field patterns for moving domains employs magnetically soft overlay patterns of Permalloy material which generate magnetic poles in response to fields in the plane of the material. The in-plane field is reoriented causing movement of the poles and, consequently, movement of the domains which are attracted by them. A pattern employing the popular T and bar shaped Permalloy overlay elements is described in U.S. Pat. No. 3,613,058, among others. Chevron shaped elements are described in U.S. Pat. No. 3,729,726 among others.
The more popular of the two systems just described is the T and bar shaped arrangement, which allows bubbles to be propagated in either direction therealong in accordance with inplane field direction. Although generally acceptable, some difficulty with the T and bar shaped Permalloy organizations have been observed. The major difficulty observed is caused by the gaps or the spacing between the soft magnetic elements. The narrower the gaps the better the circuit operates. But, the quality control problem achieving uniform and narrow gaps is extremely serious. A second problem is caused by the corners of such Permalloy circuits. To conserve space, the loops are elongated and include sharp double back curves or corners. Although such arrangements work satisfactorily in many relatively low frequency applications, they have proven unsatisfactory at higher frequencies and higher bit densities.
An alternate means to the Permalloy propagation patterns of defining a path along which bubbles are sustained and can be moved has been developed and is known as the localized ion implantation technique. Such implantation through a patterned photoresist mask may be used to alter the magnetic anisotropy of magnetic garnets and to thereby produce rails which guide bubbles in garnet epitaxial films.
The patterns established for magnetic memory organizations using the ion implanted garnet technique have followed the well known pattern of the Permalloy arrangements. One popular family for Permalloy circuits is the major-minor loop pattern shown, for example, in U.S. Pat. Nos. 3,613,056; 3,618,054 and 3,729,726 among others. In such an organization, read and write connections are made to the major loop and data bubbles are exchanged from the minor loops and the major loop at transfer gates, where the loops come into close proximity with one another. The advantage of such an organization is that the magnetic rotating fields do not have to be reversed since all data is confined within continuous loops that provide a circulating data path. An analogy may be made to a magnetic rotating drum that carries the data again and again past the same point for data reading and writing.
Hybrid arrangements of the major-minor loops configuration, such as shown in U.S. Pat. No. 3,613,058, have continuous minor loops, but a single path to the read and write circuitry. Such organization or arrangement lends simplicity and short access to the stored data, but requires in-plane magnetic field rotation reversals, and has not had wide spread acceptance.
Ion implanted garnet circuits following the organizations of the Permalloy circuits have heretofore taken up large amounts of area on a chip. That is, the major and minor loops in the arrangement just described have heretofore been formed by stringing the non-implanted circles one after another until the string closes on itself. When it is remembered that the domain or bubble size is only a fraction of the non-implanted circular areas forming the path, it may be seen that even one loop takes up considerable space. Further, using conventional photoresist masking techniques, the masked circular areas tend to be ill-defined at their cusps, that is, at the junction points between the circular areas, as contrasted to the regions exposed to ion implantation and comprising the remainder of the surface structure of the magnetic garnet layer. Each loop of non-implanted circles is elongated and although both inside and outside surfaces of the loops are theoretically suitable for travel of the domains, only travel on the outside has been satisfactory. This is because only at the outside is loop-to-loop transfer permitted without external wiring. Moreover, outside travel is used to prevent unwanted across-the-loop transfer, since to conserve space the distance across the loop is kept quite short.
Finally, it has been thought heretofore that loop-to-loop transfer, that is, between the minor loops and the major loop, had to take place where the loops were closest to each other. This is true of the transfer gate of a T-bar Permalloy circuit. However, one shortcoming of an ion implanted garnet circuit major-minor loop arrangement has been that if the circular non-implanted area of a minor loop is directly abreast of one of the circular areas of the major loop, then at the instant when the propagating field at the edge of the minor loop is suitable to release the bubble, the field at the major loop is not of the correct phase to receive the bubble. It has been necessary, therefore to retain the bubble at the end of the minor loop for one-half of the clock period until the phase of the field at the major loop is correct to accept the bubble, a very unhandy operating phenomenon.
Therefore, it is one feature of the present invention to provide an improved embodiment of an ion-implanted garnet, major-minor loop arrangement in which the non-implanted areas are formed in rows.
Another feature of the present invention is to provide an improved embodiment of an ion-implanted magnetic bubble memory device employing non-implanted areas comprising major and minor rows in an otherwise ion-implanted magnetic garnet that provide transfer of bubbles from loop to loop wherein the releasing loop and receiving row are offset to permit bubble release and acceptance operating at the same phase of the propagating field.
Yet another feature of the present invention is to provide an improved method of masking an organization to provide contiguous, circular non-implanted areas in an otherwise ion-implanted garnet areas so that the cusps formed between in the circular non-implanted areas are sharply defined, thereby providing trouble-free high speed data propagation.
Still another feature of the present invention is to provide improved means of bubble generation, replication and annihilation in conjunction with an ion-implanted magnetic bubble memory device having a major-minor loop arrangement in which the non-implanted areas are formed in rows.